3.2 Method-Based Discriminating Functions

The method-based discriminating functions are used if all the methods of
the generic function at the time of the first call are suitable:
therefore, these discriminating function strategies do not transition
into any of the other states unless the generic function is
reinitialized. Of these discriminating functions, the simplest is the
SB-PCL::NO-METHODS, which is appropriate when the generic
function has no methods. In this case, the discriminating function
simply performs an argument count check1 and then calls
NO-APPLICABLE-METHOD with the appropriate arguments.

If all of the specializers in all methods of the generic function are
the root of the class hierarchy, t, then no discrimination need
be performed: all of the methods are applicable on every
call2. In this case, the SB-PCL::DEFAULT-METHOD-ONLY
discriminating function can call the effective method directly, as it
will be the same for every generic function call.3

If all methods of the generic function are known by the system to be
side-effect-free and return constants, and the generic function has
standard-method-combination and no eql-specialized methods, then the
SB-PCL::CONSTANT-VALUE discriminating function can simply cache
the return values for given argument types. Though this may initially
appear to have limited applicability, type predicates are usually of
this form, as in ex:pred4.